Feasibility of Seasonal Forecasts Inferred from Multiple GCM Simulations

Abstract Assuming that SST provides the major lower boundary forcing for the atmosphere, observed SSTs are prescribed for an ensemble of atmospheric general circulation model (GCM) simulations. The ensemble consists of 9 “decadal” runs with different initial conditions chosen between 1 January 1979 and 1 January 1981 and integrated about 10 years. The main objective is to explore the feasibility of seasonal forecasts using GCMS. The extent to which the individual members of the ensemble reproduce the solutions of each other (i.e., reproducibility) may be taken as an indication of potential predictability. In addition, the ability of a particular GCM to produce realistic solutions, when compared with observation, must also be addressed as part of the predictability problem. A measure of reproducibility may be assessed from the spread among ensemble members. A normalized spread index, σn/σs, can be defined at any point in space and time, as the variability of the ensemble (σn) normalized by the climatologic...